1. Field of the Invention
This invention relates to carbon electrodes made with fibrillated polytetrafluorethylene (PTFE) for electrochemical devices and their methods of preparation.
2. Prior Art
The process of fibrillating PTFE has been known for some time and has been included in many patents that will be discussed later. During fibrillation, other ingredients are added that provide various aspects of functionality such as carbon for electrical capacitance and chemical adsorption.
The typical process to produce fibrillated PTFE with carbon is to mix PTFE, carbon, and solvents under high shear and high temperature, biaxially calendar the material at high temperature, extrude into the final form at high temperature, and dry the product at high temperature to remove the solvents. It is clear that high temperature was considered a critical part of the process of creating PTFE carbon electrodes in the prior art and that the final product must be dried prior to use in devices. Any solvents that are critical to the operation of the device must be added in a subsequent operation.
It is clear from the prior art for producing carbon containing electrodes made with PTFE that the first step must be a high shear mixing step. Ree et al. (U.S. Pat. No. 4,153,661), in describing production of carbon filled PFTE webs, specifies to mix materials together and then high shear mixed between 50–100 C. prior to biaxial calendering. Solomon (U.S. Pat. No. 4,379,772) shows that materials are to be mixed without solvents, and then high shear mixed to cause fibrillation. Shia (U.S. Pat. No. 4,556,618), describes the importance of initial high shear mixing on the final properties of the carbon electrode. Finally Andelman (U.S. Pat. No. 6,127,474), while describing the manufacture of carbon electrodes for a flow through capacitor, specifically states that the ingredients are typically mixed under high heat and high pressure conditions.
The next step in the process is the biaxial calendering of the mixed material to produce a fibrillated sheet. A consistent theme in the prior art is the insistence of high temperature during this process. Ree et al. (U.S. Pat. No. 4,153,661) teaches that this step should be done from 50–100 C. Solomon (U.S. Pat. No. 4,379,772) recommends a target calendering temperature of 50 C. Andelman (U.S. Pat. No. 6,127,474) recommends 320 F. as a target temperature. The purpose of the biaxial calendering step is to fibrillate the material and produce a flexible sheet. Fibrillation occurs when the material is subjected to high shear, which causes the microscopic particles of PTFE contained in the mixture to unravel. These particles unravel similar to a ball of yarn that has many strings wrapped around it. As the many pieces of string of the many particles unravel, they begin to intertwine with each other, creating the microscopic web that holds together all of the particles and provides the sheet with tensile strength and dimensional stability. The shear rate of an operation is significantly affected by the temperature of the material being sheared. As the temperature rises, the viscosity of the material drops and the shear rate drops. By biaxially calendering material at higher temperatures, the time and effort to produce an equivalently fibrillated sheet is much longer. Also, running this operation at higher temperatures, especially those approaching the boiling point of water, causes the material to lose water quickly. As water is lost, the viscosity of the material rises in an uncontrolled manner, the rate of fibrillation increases quickly, and makes it very difficult to fibrillate to a consistent level.
The drying step is universally recommended in all prior art references. When carbon PTFE material is dried, water that had been incorporated into the very small pores within and around the carbon particles is remove as vapor. When the material is rewet, some of these originally wet internal pores do not rewet. Since these devices work on the principal of ion adsorption onto surfaces of carbon particles, the less surface available to the ion, the less adsorption. When the adsorption of water is reduced, the overall capacitance of the device is reduced. Also, the electrical resistance of the device increases. This is due to the lack of open pathways for the ions to electrochemically diffuse into and out of the carbon electrodes. When this electrochemical diffusion resistance is higher, the speed at which the device can operate is reduced, thereby reducing the ion removing power. In summary, drying the material reduces the capacity of the device by 10 to 15%.
Vallance (U.S. Pat. No. 3,890,417), Goldsmith (U.S. Pat. No. 3,281,511), Ree et al. (U.S. Pat. No. 4,153,661), Bernstein (U.S. Pat. No. 4,320,185), Solomon (U.S. Pat. No. 4,379,772), Shia (U.S. Pat. No. 4,556,618), Morimoto (U.S. Pat. No. 4,862,328), Hiraksutka (U.S. Pat. No. 6,072,692), and Andelman (U.S. Pat. No. 6,127,474) all describe the importance of drying carbon electrode material. There are some applications where it is important to eliminate any traces of water due to the design of the device and others where the PTFE or other binder/component must undergo sintering at high temperature. In descriptions for electrode material clearly destined for aqueous electrochemical devices, drying is a described as a key step. Ree et al. (U.S. Pat. No. 4,153,661) recommends drying at 20–100 C. for anywhere from 1 to 100 hours. Andelman (U.S. Pat. No. 6,127,474), describing the process to produce carbon electrodes for use in aqueous electrochemical devices, states as examples either formulations that do not have any water, or small amounts that then are subjected to extremely high temperatures. In any case, the resultant carbon electrodes produced by the Andelman (U.S. Pat. No. 6,127,474) patent are dry.
Not only do dried electrodes never regain their original ionic capacity, they also take an extremely long time to rewet. Studies have shown that it takes anywhere from a few days to a few weeks for a dried electrode to come to equilibrium. This equilibrium level is not equivalent to the original absorption capacity as mentioned above.
It is critical to prevent active surface area from drying due to the electrode's inability to rewet and hence loss performance. In order for the wetted surfaces to function in a device, ions must be originally present in the water that fills the porosity. If this water does not contain a sufficient amount of ions or conductivity, upon assembly, the ionic capacity of the device is reduced and can never recover. The reason that ions can not diffuse into these deionized areas after assembly is due to the fact that the carbon electrode is sandwiched in between a solid current collector and an ion specific membrane in most aqueous electrochemical devices. This arrangement prevents any further ion pairs from diffusing into the electrode, hence reducing the ionic capacity of the electrode and device.